(gentle music) Black bears are an important native species here in Virginia.

Most of them stay somewhat close to one area, but they have been known to travel, sometimes as much as 95 miles in a year.

*Little bear, you can kind of see him back there, lives here at Maymont, a historic estate and park in Richmond.

But the natural areas where black bears live are getting smaller each year due to development and pollution.

This means that smaller areas are home to more bears than ever.

But how do we know how many of a species a habitat can support?

It's all related to a concept called carrying capacity, which is what we're exploring today on "Spotlight Earth".

It might surprise you that Virginia's black bears are omnivores.

They eat mostly plants in the spring, berries and insects in the summer, and nuts and berries in the fall.

That's not to say if they come upon a dead animal, they won't eat it.

But as development impedes on more and more of their native habitats, bears are finding it much more difficult to find food and other resources necessary for them to live healthy and happy lives.

My "Spotlight Earth" co-host Hales is living happy and healthy in her natural habitat, the "Spotlight Earth" studio.

She joins us to explore the concept of carrying capacity.

Hales, take it away.

Thanks, Jarrell.

Carrying capacity is the number of living organisms that a region can support without environmental degradation.

We know that many ecosystems exist in a delicate balance.

They usually have just enough abiotic and biotic resources to support the organisms that live there.

That means that the soil has the right amount of nutrients, the sun provides enough light, and there are enough primary producers and low level consumers to support the top predators.

A healthy ecosystem evolves into a balanced community of interacting populations.

The more diverse the community, the more resilient the ecosystem.

Simpler systems like the Arctic Tundra have a harder time rebounding from disturbances.

Available food and water supply are important limiting factors.

Limiting factors are an ecosystem's biotic and abiotic aspects that can limit the size or distribution of a population or species.

Other important limiting factors are available mates, space, sunlight, and temperature.

If any of these is disturbed in an ecosystem, it can harm one or more species.

These limiting factors have specific minimum and maximum values known as tolerance limits.

Beyond these tolerance limits, a population or species cannot survive or mate.

Carrying capacity is the amount of a species that a habitat or ecosystem can support before it degrades or depletes.

Let's consider our friend the black bear in Virginia.

To support a stable species like the black bear, a habitat must have enough berries, insects, nuts, and animal meat.

If natural or human disruption eliminates one of these food sources, it will impact the number of bears that the habitat is able to support and change the carrying capacity of black bears.

Let's check back in with Jarrell at Maymont to learn more.

(upbeat music) Thanks, Hales.

To learn more about carrying capacity, I've enlisted the help of an expert.

This is Krista Weatherford.

She's the director of programming and community engagement here at the Robins Nature Center at Maymont.

We're here at the Otter Exhibit to learn a little bit more about carrying capacity.

Can you tell us a bit more about the North American river otter?

North American otters are very playful creatures that we find in and around a lot of rivers throughout the United States, and they're here in Virginia.

Their prey sources are fish.

They also like a lot of reptiles, amphibians, so frogs and snakes and things like that.

They'll eat birds, bird eggs.

Let's say there's a shortage in the mussel population.

How would that impact the habitat's carrying capacity for the river otters population?

Obviously, if a preyed species declines, then we're going to see the otter species or the predator species that will also decline.

Right?

It may take a little bit of time, maybe by the end of the year or the following year, that you would actually see that decline.

There's, you know, natural fluctuations in populations, and so that predator population is gonna follow whatever the prey population is doing.

I mean, that seems really serious.

(chuckles) It can be.

We need to remember that these fluctuations are normal in ecosystems.

It's just part of the ecosystem, but it is important for wildlife managers to keep an eye on that, so that if there are detrimental things happening within an ecosystem, they can get a handle on it, and hopefully those populations can recover then afterward.

That's great information.

Krista.

Thank you so much for giving us a tangible example of carrying capacity.

Time to head back to the studio to explore these population curves.

Hales, ready to do some graphing?

Ready, Jarrell.

Oh, that otter is peaking on the cuteness quotient chart, by the way.

The population changes of a species can be represented through population curves.

There are two types of population growth patterns, exponential and logistic.

When resources are unlimited and there are limited predators, species may experience exponential growth.

Let's look at this graph.

We have time on the horizontal or x axis, which can also be represented as the generation number.

On the vertical or Y axis, we have the number of individuals in the population or species.

At first, there is small growth because there are a limited number of reproducing individuals.

After a few generations, there are more reproducing individuals, and the population really begins to rise.

As population numbers increase, the rate of growth increases, and we can see an exponential or J shaped curve.

This type of growth tends to happen in very small populations, or in regions that are newly colonized by a species.

But we live in reality and know that an ecosystem or habitat will probably not have unlimited resources or zero predators for a species.

Remember all those limiting factors we discussed?

When a population grows exponentially and reaches its carrying capacity, the amount of growth tapers off, and you get this S-shaped curve.

As some of the individuals die off, the population might rise again, but only to that carrying capacity before it dips down slightly again.

So to wrap up, using these graphs is a quick way of looking at what a population is doing.

Nature evolves to balance itself.

Through succession, ecosystems maintain balance and restore equilibrium.

Animals and plants will thrive in an environment with abundant resources, but the powerful interactions and limiting factors that drive evolution will determine the distribution and number of the species in the ecosystem.

Every system has its carrying capacity, even for us humans here on the planet.

Thanks for watching.

See you next time on "Spotlight Earth".

(upbeat music) (upbeat music continues)